Power semiconductor devices, such as power diodes, power MOSFETs, or power IGBTs, are designed to withstand high blocking voltages. Those power devices include a pn-junction that is formed between a p-doped semiconductor region and an n-doped semiconductor region. The device blocks (is switched off) when the pn-junction is reverse biased. In this case a depletion region (space charge region) propagates in the p-doped and n-doped regions. Usually one of these n-doped and p-doped semiconductor regions is more lightly doped than the other one of these semiconductor regions, so that the depletion region mainly extends in the more lightly doped region, which mainly supports the voltage applied across the pn-junction.
The ability of a pn-junction to support high voltages is limited by the avalanche breakdown phenomenon. As a voltage applied to a pn-junction increases, an electric field in the semiconductor regions forming the pn-junction increases. The electric field results in an acceleration of mobile charge carriers present in the semiconductor region. An avalanche breakdown occurs when, due to the electric field, the charge carriers are accelerated such that they create electron-hole pairs by impact ionization. Charge carriers created by impact ionization create new charge carriers, so that there is a multiplication effect. At the onset of an avalanche breakdown a significant current flows across the pn-junction in the reverse direction. The voltage at which the avalanche breakdown sets in is referred to as breakdown voltage.
The electric field at which the avalanche breakdown sets in is referred to as critical electric field (Ecrit). The absolute value of the critical electric field is mainly dependent on the type of semiconductor material used for forming the pn-junction, and is weakly dependent on the doping concentration of the more lightly doped semiconductor region.
The critical electric field is a theoretical value that is defined for a semiconductor region that has an infinite size in directions perpendicular to field strength vectors of the electric field. Power semiconductor devices, however, have a semiconductor body of finite size that is terminated by edge surfaces in lateral directions. In vertical power semiconductor devices, which are semiconductor devices in which the pn-junction mainly extends in a horizontal plane of the semiconductor body, the pn-junction usually does not extend to the edge surface of the semiconductor body but is distant to the edge surface of the semiconductor body in a lateral direction. In this case, a semiconductor region (edge region) of the semiconductor body adjoining the pn junction in the lateral direction also has to withstand the blocking voltage.
In the edge region, an edge termination structure can be implemented that helps to improve the voltage blocking capability in the edge region. Different types of edge termination structures are known. One of those edge termination structures includes a doped field ring that surrounds the semiconductor region with the pn junction and that is connected to a field plate.